The invention relates to a method of producing high-purity metal powder by the electron beam melting of bars of material in a vacuum, momentarily catching the molten material on a plate spinning at high speed from which particles of the material are flung and then soldified by the removal of heat.
Metal powders are required for a number of purposes, of which only sintered metal products and surface coatings will be mentioned here by way of example. A number of superalloys can be produced with satisfactory material characteristics only if they are made from powdered metals. In order to achieve optimum properties in the finished product it is necessary that the metal powder that is produced have a very precise particle size spectrum. In addition, the metal powder must be produced in a very pure state and must contain no products of reaction with atmospheric oxygen or other reactive gases when it enters the sintering process. Hollow spheres and foreign substances in and between the particles are to be avoided. In particular the powder particles must be free of oxide coatings. To satisfy this requirement, the electron beam heating system must be provided with the high vacuum of 10.sup.-4 bar and less that is needed for the unhampered flight of the electrons.
As a consequence of the application of the vacuum, however, the transition from the molten to the solid state, i.e., the removal of the melting heat, can be brought about exclusively by radiation losses during the free flight of the metal particles, unless other, serious disadvantages can be accepted. The removal of heat by convection and conduction is impossible, as is also the use of a cooling and quenching liquid within the evacuated chamber. The removal of heat by radiation must result in the solidification of the metal particles before they contact one another or any solid object. If the metal particles touch one another they cake together, and if they contact some other solid object they become flattened and this is undesirable for most applications. These circumstances necessitate flight paths of considerable length. On the other hand, short flight paths are desirable on account of the necessary dimensions of the vacuum chambers, on whose volume depends not only the time required for evacuation but also the choice of the pump system. But in most cases the required particle size is specified and this precludes flexibility as regards the size of the powder producing apparatus.
German "Auslegeschrift" 1,291,842 discloses a method of producing metal powder by means of electron beams, in which the rod which is to be made into powder is itself rotated at high speed. The end of the rod is bombarded with electron beams such that molten particles are flung outwardly by centrifugal force. On account of the relationship between the diameter of the rod and the flight path or cooling rate of the particles, and hence the volume of the vacuum chamber, the spinning rod can have only a limited diameter. For a specific amount of the powder, therefore, the apparatus has to be fed either with a rod of sufficient length or with a number of short rods. Reloading necessitates shutting down the powder making system and long, thin rods, due to unavoidable instabilities, cannot be spun at sufficiently high speed since it is impossible to support them over their entire length. Furthermore, the cooling of the metal particles is accomplished at least partially by impact and the removal of heat on a cooled surface, so that the shape of the solidified metal particles departs from the spherical. Lastly, however, the metal particles are flung away from the spinning rod in all directions, so that the cooled wall must be rotationally symmetrical with respect to the rod. The principal disadvantage is that, in addition to the above-indicated disadvantages as regards the quality of the powder, the vacuum chamber must possess a rather large volume.
German Pat. No. 1,280,501 and German "Auslegeschrift" No. 1,565,047 furthermore disclose methods for the production of metal powder by electron bombardment, in which the molten metal drops onto the vibrating surface of a receiver vibrating at high or ultrasonic frequency. The production capacity of such an installation, however, is very limited, since the vibrating receiver can be fed only a very small amount of molten metal per unit of time. Also, metal powders having a broad particle size distribution are produced. Above all, however, the vibrating receiver propagates the metal particles in uncontrolled directions, so that the receiver must be located substantially in the center of the vacuum chamber which must be dimensioned accordingly. The flight paths of the metal particles in all directions again determine the size and shape of the vacuum chamber. Premature collision between the still hot metal particles results in a caking or sintering of the particles.
Lastly, German "Auslegeschrift" No. 1,783,089 discloses a process of the initially described kind, in which the molten metal impinges upon a plate which is spinning at a high speed. In this case, again, the metal particles produced by centrifugal force are flung from the entire circumference of the plate. Solidification by removal of heat is accomplished in this case by a cooling jacket surrounding the spinning plate in very close proximity thereto, so that the early impingement of the molten particles upon this cooling jacket results in the production of virtually naught but flake-like granules. Even so, the volume of the vacuum chamber cannot be reduced to the desired extent.